Process for providing a tortoise-shell chromatic effect to metallic substrates

ABSTRACT

The present invention relates to a chemical process for providing a tortoise-shell chromatic effect to nickelated metallic substrates, which comprises heating the metallic substrate to 100-110 ° C. and subsequent buffering at a temperature of 40-110° C. with an oxidizing solution based on sodium thiosulfate, a reagent metal and an acidifying compound.

[0001] The present invention relates to a process for providing atortoise-shell chromatic effect to metallic substrates.

[0002] In particular, the present invention relates to a process forproviding a tortoise-shell chromatic effect to nickelated components offire-arms.

[0003] It is known that the so-called tortoise-shell effect is aparticular spotted chromatic effect conferred for ornamental purposes totools or metallic parts of devices of varying origins.

[0004] In the fire-arm industry, the tortoise-shell effect isparticularly requested for providing an aesthetically pleasant chromaticeffect to the action body or other metallic parts of shotguns.

[0005] A tortoising process of metallic substrates which requires highoperating temperatures and also a low temperature chemical processcapable of providing a simple uniform colouring to metallic substrates,are currently known.

[0006] The high temperature tortoising process consists in the thermaltreatment of a steel matrix which comprises heating to a temperature ofabout 700° C. and quenching the steel in an aqueous solution ofoxidizing salts.

[0007] The spotted colouring is created by the different surfaceoxidation degree of the steel and by the different thickness of theoxide layer formed.

[0008] This tortoising process of the known art, however, is not withoutits drawbacks which can mainly be attributed to the distortion of themetallic materials due to the considerable thermal stress to which theyare subjected during the quenching operation.

[0009] The more reduced the dimensional tolerances of the metallicarticles or components treated, the greater this problem becomes.

[0010] In the case of the treatment of metallic components forhigh-precision articles, in fact, even a limited distortion or almostimperceptible deformation of the component can jeopardize its adequacyfor the end-use.

[0011] In the tortoising processes of gun action bodies, for example, ithas been found that even minimum structural distortions can make themunusable.

[0012] Furthermore, the thermal treatment required for obtaining theparticular aesthetic tortoise-shell effect has a negative influence onthe hardness and mechanical performance in general of the metalliccomponent treated.

[0013] Viceversa, chemical colouring processes of metals at a lowtemperature not only have the disadvantage of not being suitable forproviding a spotted chromatic effect on the metals but also have theadditional drawback that the chromatic layer produced can be easilyremoved by simple scratching.

[0014] With the tortoising techniques currently available, it isconsequently not possible to obtain a tortoise-shell chromatic effectwhich persists with time, is resistant to scratching and does notproduce significant distortions of the metallic end-products treated.

[0015] The Applicant has now identified a chemical tortoising processwhich is effected at non-high temperatures and which consequentlyconsiderably limits the occurrence of the drawbacks described above.

[0016] One of the main objectives of the present invention thereforerelates to providing a process for giving a tortoise-shell chromaticeffect to metallic substrates without having to resort to temperatureswhich cause distortion of the metals treated.

[0017] Another objective of the present invention relates to a chemicaltortoising process of end-products or metallic components which allowsthe production of a long-lasting chromatic effect, which is appreciablefrom an aesthetic point of view.

[0018] A further objective of the present invention relates to achemical process capable of providing, at non-high temperatures, apersistent tortoise-shell chromatic effect to nickelated components offire-arms.

[0019] In view of the above objectives and others which will becomeevident from the following description, a first aspect of the presentinvention relates to a process for providing a tortoise-shell chromaticeffect to substrates which includes the heating of a metallic substrateand application, under heat, on a portion of said metallic substrate, ofan oxidizing solution comprising a thiosulfate, a reagent metal and anacidifying compound.

[0020] In the process of the invention, the metallic substrate to betreated is heated to a temperature at which there are substantially nothermal distortions which can jeopardize its use in high-precisionapplications.

[0021] The oxidizing solution of the invention is typically a solutionbased on sodium thiosulfate containing a reagent metal conveniently inthe form of an acetate, preferably consisting of copper or lead acetateor their mixtures.

[0022] Within the scope of the present invention, the term acidifyingcompound includes compounds with acid hydrolysis or however compoundswhich when added to an aqueous solution are capable of reducing its pH.

[0023] Suitable acidifying compounds comprise organic acids such ascarboxylic acids, for example, acetic acid, citric acid; oxalic acid,and potassium acid tartrate; inorganic acids such as hypophosphorousacid; salicylic acid; ketones such as acetone; aldehydes such as formicaldehyde and acetic aldehyde and their mixtures.

[0024] Substrates suitable for being subjected to the tortoising processof the invention comprise metallic and non-metallic matrixes, providedthey have at least one surface portion with a metallic layer,galvanically deposited or with another technology. Plastic matrixes witha nickelated surface, steel matrixes, nickelated steels, metal alloyssuch as aluminum, copper, nickel alloys and super-alloys, can be used,for example.

[0025] Nickelated details or components of end-products and articles forvarying uses, with particular reference to nickelated details forfire-arms, such as gun action bodies, can be appropriately treated withthe process of the invention.

[0026] The application of said oxidizing solution advantageously takesplace by means of buffering on localized portions of the metallicsubstrate to be treated so as to cause oxidation and consequently achromatic variation, limited to the buffered areas. In the areasbuffered with the oxidizing solution, the surface metal layer undergoesa more intense oxidation process the greater the residence time of theoxidizing solution.

[0027] An embodiment of the process of the invention comprises thedirect development of the chromatic effect on the metallic substrateusing a solution based on thiosulfate containing copper acetate asreagent metal.

[0028] In accordance with this embodiment, a metallic substrate isheated to a temperature conveniently within the range of 100 to 110° C.and subsequently buffered with a chromatically effective quantity of anaqueous solution containing sodium thiosulfate, copper acetate andpotassium acid tartrate, at a temperature advantageously within therange of 40-110° C., preferably 60-80° C., until the development of thedesired chromatic variation. The buffering phase can be repeated once orseveral times, until the desired tonality or chromatic effect isobtained.

[0029] It has been found that under these conditions, the tortoise-shellchromatic effect is caused by the oxidizing action of the solution onthe surface of the metallic substrate. The coloured oxidized layerformed is firmly anchored to the metallic surface producing along-lasting chromatic effect.

[0030] The Applicant has discovered that the direct development of along-lasting chromatic variation which is particularly appreciable froman aesthetic point of view, is obtained using an oxidizing aqueoussolution containing 220-240 g/l of sodium thiosulfate, 30-40 g/l ofcopper acetate and 30-40 g/l of potassium acid tartrate.

[0031] According to another embodiment of the process of the invention,it is possible to obtain the formation of the tortoise-shell effect witha two-phase treatment. This further embodiment of the inventiontherefore includes:

[0032] a first treatment phase which comprises the heating of a metallicsubstrate to a temperature conveniently within the range of 140-150° C.and the application of a chromatically effective quantity of an aqueoussolution containing sodium thiosulfate, lead acetate and an acidifyingcomponent, preferably consisting of potassium acid tartrate and, withoutrinsing;

[0033] a second treatment phase which comprises the application of asecond oxidizing solution to the substrate, conveniently brought to atemperature of 40-80° C., preferably 50-60° C. The application of saidsecond oxidizing solution can be effected by means of buffering or bythe direct immersion therein of the metallic substrate.

[0034] This second oxidizing solution contains a strong oxidizing agent,preferably consisting of ammonium persulfate, preferably present in aquantity ranging from 150-180 g/l.

[0035] It has been found that the process of the invention allows avariety of chromatic effects and different colourings to be obtained, bysimply increasing the contact time of the oxidizing solutions on thesurface of the metallic substrates. It is thus possible to produceyellow-gold, red, blue and grey colourings and shades in differenttonalities.

[0036] The following examples are provided for purely illustrativepurposes of the present invention and should not be considered aslimiting its protection scope which is clearly defined by the enclosedclaims.

EXAMPLE 1

[0037] A nickelated steel action body for a shotgun was initially heatedto a temperature of 100° C. and subsequently subjected to localizedbuffering at a temperature of 70° C. with an oxidizing aqueous solutioncontaining 230 g/l of sodium thiosulfate, 35 g/l of copper acetate and35 g/l of potassium acid tartrate, until a spotted colouring, of thetortoise-shell type, is formed.

EXAMPLE 2

[0038] An aqueous solution containing 230 g/l of sodium thiosulfate, 22g/l of lead acetate and 25 g/l of potassium acid tartrate was appliedwith a buffer on a nickel alloy article heated to a temperature of 140°C. In a subsequent phase, the article was immersed or buffered, withoutrinsing, at a temperature of 60° C., with an aqueous solution containing160 g/l of ammonium persulfate. After a few minutes, a chromaticvariation, with a typical tortoise-shell effect, is formed in thebuffered areas.

EXAMPLE 3

[0039] Oxidizing solutions suitable for use in the buffering phase ofthe tortoising process of the invention: Sodium thiosulfate 220/240 g/lCopper or lead acetate 25-40 g/l Citric acid 30-40 g/l Sodiumthiosulfate 220/240 g/l Copper or lead acetate 25-40 g/l Hypochlorousacid 0.5 cc/l (d = 1, 23 g/cc) Sodium thiosultate 220/240 g/l Copper orlead acetate 25-40 g/l Acetone 40 ml/l Sodium thiosulfate 220/240 g/lCopper or lead acetate 25-40 g/l Formic aldehyde 30 ml/l Sodiumthiosulfate 220/240 g/l Copper or lead acetate 25-40 g/l Acetic aldehyde20-30 ml/l Sodium thiosulfate 220/240 g/l Copper or lead acetate 25-40g/l Oxalic acid 10-15 g/l Sodium thiosulfate 220/240 g/l Copper or leadacetate 25-40 g/l Salicylic acid 15-20 g/l Sodium thiosulfate 220/240g/l Copper or lead acetate 25-40 g/l Acetic acid 10 ml/l

1. A process for providing a tortoise-shell chromatic effect tosubstrates which comprises heating of the substrate to a suitabletemperature for providing a chromatic effect and the application underheat on a portion of said substrate of an oxidizing solution comprisinga thiosulfate, a reagent metal and an acidifying compound.
 2. Theprocess according to claim 1, characterized in that said thiosulfate issodium or potassium thiosulfate.
 3. The process according to claim 1,characterized in that said metal is selected from copper acetate, leadacetate or their mixtures.
 4. The process according to claim 1,characterized in that said acidifying compound is selected frompotassium acid tartrate, citric acid, hypophosphorous acid, acetone,formic aldehyde, acetic aldehyde, oxalic acid, salicylic acid, aceticacid and their mixtures.
 5. The process according to claim 1,characterized in that said oxidizing solution is applied by buffering onsaid substrate.
 6. The process according to claim 1, wherein there isthe direct development of a tortoise-shell chromatic effect by heatingsaid metallic substrate to a temperature within the range of 100-110° C.and buffering at a temperature within the range of 40-110° C. with anoxidizing solution comprising sodium thiosulfate 220-240 g/l copperacetate 30-40 g/l potassium acid tartrate 30-40 g/l

until the chromatic effect is obtained.
 7. The process according toclaim 6, characterized in that the buffering of the metallic substratetakes place at a temperature ranging from 60-80° C.
 8. The processaccording to claim 1, wherein a tortoise-shell chromatic effect isdeveloped in two phases, said process comprising: an application phaseby buffering a solution comprising sodium thiosulfate 220-240 g/l leadacetate 30-40 g/l potassium acid tartrate 30-40 g/l

on a metallic substrate heated to a temperature ranging from 140-150° C.and a second application phase of a second oxidizing solution on saidsubstrate at a temperature ranging from 40-80° C.
 9. The processaccording to claim 8, characterized in that in said second phase, theapplication of the oxidizing solution takes place by buffering or byimmersion of the metallic substrate.
 10. The process according to claim8, characterized in that said second oxidizing solution is a solution ofammonium persulfate.
 11. The process according to claim 1, characterizedin that said substrate comprises a metal coating.
 12. The processaccording to claim 11, characterized in that said coating metal isselected from nickel, nickel alloys, steels, nickelated steels, copper,super-alloys.
 13. The process according to claim 1, characterized inthat said metallic substrate is an action body of a gun.